Excel 2007 : Adding Time and Duration

Hello,

I am trying to add two time values and have thus far been unsuccessful:

The first value is a "Start Time" which can be expressed as 13:00 (military time = 1 PM).

The Second Value is the duration which is expressed in seconds - 3600 (as in 3600 seconds = 60 minutes).

What I would like to calculate is the "Stop Time" for this event.

To Elaborate: Start Time + Duration = Stop Time
For this example the expected result would be 13:00 + 3600 seconds = 14:00

I have tried every time formatting variation I could think of, however the "duration" value is always displayed as a time and not a duration.

Any help with this will be greatly appreciated.